1. Field of the Invention
The present invention relates to active pixel sensor cells and, more particularly, to a color active pixel sensor cell with an oxide filter.
2. Description of the Related Art
Conventional imaging circuits rely on photo-diodes to convert a pixel of light energy into an electrical charge that represents the intensity of the light energy. In general, the light energy varies the electrical charge in a manner which is proportional to the photon absorption rate.
FIG. 1 shows a cross-sectional diagram that illustrates a conventional photodiode 10. As shown in FIG. 1, photodiode 10 includes an n+ region 14 which is formed in a p-type substrate 12, and isolated by adjoining regions of field oxide FOX. In addition, a microlens 16 is typically formed over the surface of n+ region 14 to direct more photons to photodiode 10.
In operation, n+ region 14 is initially reverse-biased with respect to p-type substrate 12 by applying a positive voltage to n+ region 14 while grounding p-type substrate 12. Following this, n+ region 14 is floated by removing the positive voltage from n+ region 14.
Next, light energy, in the form of photons, strikes photodiode 10, thereby creating a number of electron-hole pairs in p-type substrate 12 and n+ region 14. The electrons formed in p-type substrate 12 which diffuse to the p-n junction are swept to n+ region 14 under the influence of the junction electric field, while the holes formed in substrate 12 are attracted to ground. Similarly, the holes formed in n+ region 14 which diffuse to the p-n junction are swept to p-type substrate 12, while the electrons formed in n+ region 14 remain in region 14.
Thus, with the addition of each photogenerated electron in n+ region 14, the voltage on n+ region 14 is correspondingly reduced. As a result, photodiode 10 varies the voltage on n+ region 14 in a manner which is proportional to the photon absorption rate.
In a black-and-white imaging system, each pixel is defined by the light energy that is collected by a single photodiode 10. On the other hand, each pixel in a color imaging system is defined by the light energy that is collected by three photodiodes 10 where one photodiode 10 only collects red light, one photodiode 10 only collects green light, and one photodiode 10 only collects blue light.
FIG. 2 shows a cross-sectional diagram that illustrates a portion of a conventional color active pixel sensor cell 20. As shown in FIG. 2, cell 20 includes three photodiodes 10-1, 10-2, and 10-3 which are structurally similar to photodiode 10 of FIG. 1 and, as a result, utilizes the same reference numerals to designate the common structures.
In addition, cell 20 also includes a layer of first polyimide resin 22 which is formed over photodiode 10-1, a layer of second polyimide resin 24 which is formed over photodiode 10-2, and a layer of third polyimide resin 26 which is formed over photodiode 10-3.
Each of the polyimide layers 22, 24, and 26, which function as color filters, are dyed so that, for example, first resin layer 22 only passes red light, second resin layer 24 only passes green light, and third resin layer 26 only passes blue light. As a result, photodiode 10-1 collects only red photons, photodiode 10-2 collects only green photons, and photodiode 10-3 collects only blue photons.
One problem with cell 20, however, is that the layers of polyimide resin 22, 24, and 26 attenuate between 10% and 50% of the photons which are to be passed by the filters, depending on the wavelength of the light. Thus, there is a need for a color active pixel sensor cell that reduces the number of to-be-passed photons that are attenuated by the filter.